This invention relates in general to electrostatography and, more specifically, to an improved electrostatographic imaging member and process for fabricating the member.
Electrostatographic imaging members are well known. Typical electrophotographic imaging members include photosensitive members (photoreceptors) that are commonly utilized in electrophotographic (xerographic) processes in either a flexible belt or a rigid drum configuration. The electrophotographic imaging member may also be a flexible intermediate transfer belt. The flexible belt may be seamless or seamed. These belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate layer, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoating layer. This photoreceptor usually comprises an anticurl back coating on the side of the substrate opposite the side carrying the charge generating layer and a charge transport layer. The anticurl back coating prevents the photoreceptor from spontaneously curling after application of the above described coatings, particularly the charge transport layer. Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. Moreover, a belt exhibiting curl in highly sophisticated precision copiers, duplicators, facsimile, and the like machines leads to print and machine performance issues such as image print defects characterized by non-uniform density, contact between the curled edges of the photoreceptor with adjacent subsystems such as charging, and development subsystems which result in damage to both the belt and the subsystems, difficulty in photoreceptor belt loading, and the like. Typical adjacent subsystems include, for example charging subsystems, imaging subsystems, developing subsystems, cleaning subsystems, and the like.
There is also a great need for long service life flexible belt photoreceptors. Imaging members are generally exposed to repetitive electrophotographic cycling which subjects the exposed anticurl back layer thereof to abrasion, chemical attack, heat, multiple exposures to light, and the like. With repetitive cycling, the multilayered electrophotographic imaging belt structure gradually curls "up" at the edges to shorten service life even where other components of the belt would otherwise still perform satisfactorily. It is hypothesized that over time loss of residual solvent occurs from within the device and wear of the anticurl backing layer causes the multilayered electrophotographic imaging belt structure to eventually curl "up" at the edges. This leads to print and machine performance issues described above.
Numerous techniques have been devised to form a layer of a coating composition on a substrate. One of these techniques involves the use of an extrusion die from which the coating composition is extruded onto the substrate. For fabrication of web type, flexible electrophotographic imaging members, the extrusion die must lay down very thin coatings meeting extremely precise, critical tolerances in the single or double digit micrometer ranges. Moreover, a plurality of dies may be needed to lay down extruded coatings conventionally employed for flexible electrophotographic imaging members. The flexible electrophotographic imaging members may also comprise additional coatings applied by non-extrusion coating techniques so that the finished electrophotographic imaging member can contain as many as 5 different coatings. The extrusion die usually comprises spaced walls, each having a surface facing each other. These spaced walls form a narrow, elongated, passageway. Generally a coating composition is supplied by a reservoir to one side of the passageway and the coating composition travels through the passageway to an exit slot on the side of the passageway opposite the reservoir. Dams are provided at opposite ends of the passageway to confine the coating composition within the passageway as the coating travels from the reservoir to the exit slot. The surface of the dams facing the coating composition is generally perpendicular to the exit slot.
These extrusion dies form coatings having a uniform thickness. Where, on a single underlying surface, a coating layer of one composition is to be formed adjacent to a coating layer having a different coating composition, two different dies may be arranged side by side (e.g. see U.S. Pat. No. 4,521,457 and U.S. Pat. No. 5,614,260) to simultaneously extrude the two different compositions. This technique forms side by side coatings having a distinct junction where side by side coatings contact each other.